As a related-art motor drive control apparatus, there is given one having a structure in which a DC bus voltage for driving an inverter is generated by a rectifier circuit from an AC source such as a commercial power source. Such a motor drive control apparatus is used as, for example, a driving source for a motor of a compressor of an air-conditioning apparatus.
For such use, the motor tends to be designed such that, for the purpose of improving energy consumption efficiency in rated operation, an induced voltage (counter-electromotive force) and a power supply voltage have similar values around a rated rotation speed of the motor. Then, at the time of an overload operation in which the motor is operated at a rotation speed higher than the rated rotation speed of the motor, an output voltage of an inverter circuit is saturated to increase an output current, which leads to a reduction in operation efficiency of the motor, a reduction in operation efficiency of the inverter circuit, and other negative results.
In view of this, in order to suppress such a reduction in operation efficiency, a motor drive control apparatus including a booster circuit is proposed (for example, see Patent Literature 1). The booster circuit is connected between a rectifier circuit and an inverter circuit, and includes a reactor, a backflow preventing diode, and a switching element. A DC bus voltage rectified by the rectifier circuit is boosted by the booster circuit. In the booster circuit, energy is stored in the reactor in ON period of the switching element and the stored energy is discharged in OFF period of the switching element, to thereby boost the DC bus voltage. The boosting of the DC bus voltage in the booster circuit is controlled based on a period of time during which the switching element is turned on (on-duty). Through the boosting of the DC bus voltage in the booster circuit, a voltage to be applied to the motor increases to suppress a current to be applied to the motor, with the result that improvement in operation efficiency and expansion of the operating region are realized.
Moreover, in such a motor drive control apparatus, due to a circuit loss caused when the switching element is driven during the boosting operation of the booster circuit, the operation efficiency is reduced. For this reason, the booster circuit performs the boosting operation only in an operating region in which boosting is required.